This research will investigate the role of proliferation in thyroxin-induced differentiation of the anuran liver during metamorphosis. Treatments will include spontaneous metamorphosis, thyroidectomy, thyroxin administration, and stimulation by other mitogenic and growth promoting substances, in order to determine the specificity of thyroxin effects. Synthesis and accumulation of nucleic acids and proteins, specifically the enzymes of the urea cycle, will be monitored biochemically, and will be correlated with cell-cycle data obtained by autoradiography. Metabolic inhibitors will be used to investigate thyroxin effects on the cell cycle, to determine which thyroxin-induced responses occur in a fixed population, which are proliferation associated, and which are proliferation dependent.